A bandgap reference circuit provides a stable, precise output reference voltage for use in various analog circuits. The bandgap reference circuit is typically used in large integrated circuits for applications such as telecommunications. Bandgap reference circuits preferably provide a continuous reference voltage. It is also desirable for the output reference voltage to remain stable over varying operating conditions, such as temperature and manufacturing process variations. Recently, it has become necessary for many commercial integrated circuits to operate at less than the conventional five-volt power supply voltage, such as at three volts. Thus, bandgap reference circuits must operate over a power supply voltage range from over five volts down to three volts and less. The output reference voltage provided by known bandgap reference circuits, however, typically varies somewhat with respect to one or more of these factors. Known bandgap reference circuits also typically fail to function when the power supply voltage is lowered to three volts.
One method of providing a voltage reference is to provide a stable reference current through a precision resistor. Stable reference current circuits are known in the art. The reference current circuits may provide reference voltages which are applied to the gates of transistors in other circuits to reproduce the reference current. For example, a common type of current reference circuit provides a voltage generally designated as "NBIAS". NBIAS, when applied to the gate of an MOS N-channel transistor, produces a gate-to-source voltage which biases the transistor to have a relatively-constant drain-to-source current over wide variations in drain-to-source voltage. Thus NBIAS can be applied to the gate of an N-channel transistor whose drain is connected to a precision resistor, to provide the voltage reference.
There are at least two problems with this approach. First, known current reference circuits can produce bias voltages to reproduce currents which are suitably precise for circuits such as differential amplifiers, yet too variable for bandgap reference voltage circuits. The variability may be tolerable at higher power supply voltages, but become intolerable at lower voltages, such as at three volts. Furthermore, a known type of precision resistor available in MOS integrated circuit processing technology utilizes a specified amount of polysilicon. However, since the magnitude of current of a typical N-channel MOS transistor biased into saturation is small, and the resistivity of polysilicon is relatively small, the amount of polysilicon required to provide a suitable voltage drop for a bandgap reference voltage is quite large. Thus, valuable integrated circuit area is consumed.